best way to select edge to use bmesh.ops.bevel
I tried thes Lines: 

import bmesh
from bpy import context as C

m = C.active_object.data
bm = bmesh.from_edit_mesh(m)

indices = [0,1]
for e in bm.edges:
    if e.verts[0].index in indices and e.verts[1].index in indices:
        e.select = True
        break


The selection doesn't show up in edge select mode, but if you go to vert select, and back to edge, the edge will be correctly displayed as selected. 
How to continue from there with the operator, I have not tested.